Multi-layer approach for optimizing ferroelectric film performance

ABSTRACT

A multi-layer ferroelectric thin film includes a nucleation layer, a bulk layer, and an optional cap layer. A thin nucleation layer of a specific composition is implemented on a bottom electrode to optimize ferroelectric crystal orientation and is markedly different from the composition required in the bulk of a ferroelectric film. The bulk film utilizes the established nucleation layer as a foundation for its crystalline growth. A multi-step deposition process is implemented to achieve a desired composition profile. This method also allows for an optional third composition adjustment near the upper surface of the film to ensure compatibility with an upper electrode interface and to compensate for interactions resulting from subsequent processing.

RELATED APPLICATIONS

This application is a continuation-in-part of my application, which isassigned to the present assignee, having the same title as thisapplication, Ser. No. 09/064,465, which was filed on Apr. 22, 1998, nowU.S. Pat. No. 6,090,443, which was a continuation-in-part of my thenapplication also assigned to the present assignee, Ser. No. 08/896,684,which was filed Jul. 18, 1997, now U.S. Pat. No. 6,080,499.

BACKGROUND OF THE INVENTION

This invention relates generally to ferroelectric films. Moreparticularly, the present invention relates to a ferroelectric filmhaving improved electrical characteristics and the correspondingfabrication method for producing the ferroelectric film.

Ferroelectric films are typically used as the dielectric material for aferroelectric capacitor in a ferroelectric memory cell. For propermemory operation, it is critical that the ferroelectric film achievesdesirable electrical performance evidenced by the ability to liberate adetectable charge in response to the application of an externallyapplied electrical field. It has been shown in the laboratory thatcrystal growth characteristics and film orientation are critical toachieving this desirable electrical performance. Typically, theferroelectric film is deposited on a conducting electrode surface, suchas the bottom electrode of the ferroelectric capacitor. The bottomelectrode can be fabricated from a variety of films such as: platinum,iridium, iridium oxide, ruthenium oxide, palladium, as well as othernoble metals and their oxides, or other suitable conductive materialsknown in the art. Each of these electrodes has a unique surface withchanging roughness, and/or conductivity, which dramatically impacts thesticking coefficients for the various components of a subsequentlydeposited ferroelectric film. As the sticking coefficients vary, thecomposition of the ferroelectric film varies. As the electrode surfacebegins to accept a layer of ferroelectric film, the stickingcoefficients for the constituents change and dictate the composition ofthe bulk film. Therefore, the composition required for optimumelectrical performance cannot be obtained using a single fixed set ofsputtering conditions or solution chemistry.

For subsequent comparison to the improved ferroelectric film of thepresent invention, a typical prior art ferroelectric PZT film (leadzirconate titanate) having a thickness of about 2400Angstroms anddeposited with a single step deposition has roughly 75% of the crystaldomains oriented in the <111> crystal orientation. The “switched charge”liberated using a three-volt pulse [“Qsw(3 v)”] was measured at about 21micro-Coulombs per centimeter squared. The applied voltage at which theswitched charge of the ferroelectric capacitor is 90% saturated[“V(90%)”] was measured at about 4.5 volts.

What is desired is a ferroelectric film having optimized electricalperformance in which specific adjustments in deposition conditions aremade to compensate for the electrode variables described above and todictate a desired film composition profile.

SUMMARY OF THE INVENTION

It is, therefore, a principal object of the present invention to achievean optimized composition profile in a ferroelectric film in order toenhance electrical performance.

According to the present invention a thin nucleation layer of a specificcomposition is implemented to optimize ferroelectric crystal orientationand is markedly different from the composition required in the bulk ofthe ferroelectric film. The bulk film utilizes the establishednucleation layer as a foundation for its crystalline growth. Because ofthese composition requirements, a multi-step deposition process isimplemented to achieve the desired composition profile. The method ofthe present invention also allows for an optional third compositionadjustment near the upper surface of the film to ensure compatibilitywith an upper electrode interface and to compensate for interactionsresulting from subsequent processing.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention, which proceedswith reference to the accompanying drawing figure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a multi-layer ferroelectric filmin a ferroelectric capacitor manufactured according to the method of thepresent invention;

FIGS. 2(a), (b), (c), and (d) show the comparison of switched charge,V(90%), switched charge loss, and opposite state charge aging rate forthe two types of PZT films, one of which is annealed in an oxygenambient atmosphere, the other of which is annealed in a combined ambientatmosphere of argon and oxygen; and

FIG. 3 is a diagram of an RTA chamber for annealing wafers according tothe present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, a ferroelectric capacitor 10 is shown having abottom electrode 12, an upper electrode 20, a thin nucleation layer 14,a bulk ferroelectric layer 16, and an optional third or “cap”ferroelectric layer 18. Ferroelectric capacitor 10 can be used as thestorage element in a ferroelectric memory cell, which resides in anarray of such ferroelectric memory cells.

PZT Sputter Deposition on a Platinum Bottom Electrode Using a SingleTarget

For optimized electrical performance, the PZT ferroelectric filmmanufactured according to the present invention is sputter depositedfrom a single target with a lead-rich (“Pb”) nucleation layer 14,followed by a subsequently sputter deposited “lesser” lead-rich bulklayer 16. In order to achieve optimum electrical performance using asingle target deposition, the multi-layer approach of the presentinvention is highly desirable. To achieve a lead-rich nucleation layer14, high pressure and low power are employed to deposit a film of 50 to500 Angstroms thick with excess lead of 25 to 35%. After deposition ofthe nucleation layer 14, the sputter conditions are changed to reducethe lead content for the bulk film layer 16 (10 to 20% excess lead) bylowering pressure and raising power. These combinations of lead in thenucleation layer 14 and bulk layer 16 are critical to ensure propernucleation and film growth through the subsequent high temperatureanneals required to form the desired crystal structure. In these annealsthe volatility of lead plays a key role. The high concentrations of leadin the nucleation layer 14 are critical for ensuring an adequate leadsupply for crystal formation while maintaining an escape route for theexcess lead (i.e. the lower lead bulk layer 16). If the amount of leadin the bulk layer 16 is too excessive it will overwhelm the nucleationlayer 14 and disrupt nucleation and crystal growth. If the bulk layer 16is too lead-deficient, it consumes needed lead from the nucleation layer14 and again disrupts crystal growth.

In some cases, a lead-rich cap layer 18 is desirable to buffer filmdegradation during subsequent CMOS processing. If used, the cap layer 18is ideally sufficiently removed from the nucleation layer 14 to ensurethat it does not influence crystal growth. The cap layer 18 is optimizedfor thickness and lead content to ensure compatibility with topelectrode 20. If desired, a multi-layer bottom electrode 12 consistingof platinum and iridium, and a multi-layer top electrode 20 consistingof platinum and iridium oxide can be used. Other layers, such astitanium adhesion layers, can also be used. While platinum is apreferred electrode material, other appropriate electrode materialsknown in the art can be used as described in the Background of theInvention.

Sputtering Conditions

In the preferred embodiment, the target composition of the PZTsputtering target is as follows: 1.035 lead, 0.400 zirconium, 0.600titanium, 0.030 lanthanum, 0.050 calcium, and 0.020 strontium. Thebottom electrode 12 and top electrode 20 are platinum, each about 1750Angstroms thick.

It should be noted that the exact sputtering conditions identified beloware specific to a given sputtering tool. It is appreciated by thoseskilled in the art that the exact sputtering conditions will be modifiedas necessary to accommodate other sputtering tools.

In the preferred embodiment, a Gryphon Sputtering Deposition Tool isused. All layers 14, 16, and 18 are deposited at the same temperature(Room Temperature) and in the same sputter ambient environment (Argon).The sputter conditions for the nucleation layer 14 are 750 watts at an18 mTorr pressure to produce a film between 50 and 100 Angstroms thickwith approximately 30% excess lead. The bulk layer 16 is deposited at1000 watts at a 2 mTorr pressure to build up the remaining filmthickness with an excess lead content of about 15-20%. If an optionalcap layer 18 is employed to enhance the top electrode 20 interfacecharacteristics, the bulk layer 16 thickness is reduced by theadditional thickness of the cap layer 18. The cap layer 18 is between200 and 300 Angstroms thick and varies between 25 and 35% excess lead.The sputter conditions for producing the cap layer 18 lead contents are750 watts at a 10 mTorr pressure for 25% excess lead and 550 watts at an18 mTorr pressure for 35% excess lead.

Electrical Performance Improvement

A bi-layer ferroelectric film consisting of layers 14 and 16 has acrystal orientation of near 98% of the crystal domains oriented in the<111> orientation, with Qsw(3V) switched charge measured at three voltsof about 30 micro-Coulombs per centimeter squared. A V(90%) was measuredat about 3.5 volts. Both measurements indicate significant improvementin electrical characteristics over the single-layer ferroelectric filmreferenced in the Background of the Invention.

PZT Sputter Deposition on a Platinum Bottom Electrode Using MultipleTargets

The multiple layer sputter deposition technique described above can alsobe implemented using a “cluster tool” with a minimum of two chambers,each containing a sputtering target. In the cluster tool, a firstchamber contains a target with higher lead content. The wafer on whichthe capacitors are built is transferred to the first chamber and thenucleation layer 14 is deposited with, for example, 30% excess lead.After depositing the nucleation layer 14, the wafer is transferred to asecond chamber with an alternative target composition. The bulk layer 16is subsequently deposited in the second chamber with, for example,15-20% excess lead. An additional third chamber and target, or the firstchamber and target, can be used to deposit the cap layer 18, if desired.

Solution Chemistry or CVD (“Chemical Vapor Deposition”)

In general, the same principles of composition control, for specificregions of the ferroelectric film, apply to both solution chemistry andCVD techniques. In both cases, a composition change is required for eachof the layers. For example, in solution chemistry for the PZT exampleabove, solutions with three different lead contents and potentiallyvarying viscosities (or, at a minimum, different spin speeds) arerequired to achieve the desired film compositions and thickness targets.For CVD, the flow rates for the critical precursors are ideally changedfor each of the layers. In all cases, whether the film is crystallizedin situ or in a separate anneal step, controlling the composition of thenucleation layer 14 independent of the bulk layer 16 is critical inestablishing optimum crystal growth and orientation for ferroelectricelectrical performance.

Other Ferroelectric Materials

The techniques described herein can also be easily applied to othertypes of ferroelectric materials. One material that may have promise asa suitable ferroelectric material is strontium bismuth tantalate(“SBT”). The volatile component, or “A” site Perovskite latticeconstituent is bismuth, which, similarly to lead in the PZTferroelectric film, plays a critical role in electrical performance andthe orientation of the film. The multi-layer approach taught herein isanticipated to provide the same potential benefits regarding animprovement in electrical performance. A bismuth-rich nucleation layer14 is used, followed by a relatively bismuth-poor bulk layer 16. Ifdesired, an optional cap layer 18 can be used as in the previousexamples.

Enhancements in PZT Texturing for Low Voltage Ferroelectric Performance

According to another method of the present invention, a specific annealsequence is utilized after PZT deposition to control the crystallinetexturing of the film, and by doing so, enhancing low voltageperformance. In general, “texturing” is defined as preferred orientationand grain structure. This is accomplished by modifying the lead contentin the PZT film to take advantage of specific phase formations thatenhance PZT texturing. A multi-layer PZT is utilized to obtain aspecific lead profile, with an associated anneal sequence to takeadvantage of that profile. The anneal sequences involve a two stepprocess wherein a first step utilizes a low temperature argon ambientanneal to drive the formation of a platinum-lead and platinum-titanateintermetallic phases at the bottom electrode interface and a second steputilizes an oxygen anneal to complete the PZT crystallization process.

The multi-layer processing of the present invention is used becauseconsiderably more lead is required near the bottom electrode interfaceto supply the platinum-lead intermetallic layer; this high level of leadcannot be sustained in the bulk of the PZT film without an adverseimpact on the kinetics driving crystal formation. The argon ambientanneal is used in the first step to prevent over saturation of thebottom electrode interface with oxygen. Excessive oxygen shuts down thedesirable platinum-lead phase and favors an undesirable lead oxide (PbO)phase, which has a detrimental impact on PZT texture. By using the argonsequence of the present invention to establish the platinum-lead andplatinum titanate intermetallic phases, the energy required to produce afavorable texture during the oxygen anneal sequence can be reduced.Since the required energy is reduced, the stress/strain relationships onthe lattice are reduced and the crystal forms along preferredorientations dictated by the nucleation foundation layer. Depending uponthe PZT dopants used, the argon anneal generates a strong <111>orientation preference or a mixed texturing of <001> and <111>orientations. In particular, the quality of the <001> texturing directlyinfluences the low voltage performance of the film. An additionalbenefit of reducing the energy requirements for the anneal is theconservation of lead in the bulk of the film. This conservation of leadenhances fatigue performance.

The argon and oxygen anneal sequence of the present invention can bemodified by the addition of certain dopant materials to further enhancelow voltage performance. A PZT film can be doped with lanthanum,calcium, and strontium dopants to achieve low voltage ferroelectricoperation. Operation as low as three volts has been demonstrated, andthe potential exists for operation at even lower voltages less thanthree volts. In addition, the argon and oxygen anneal described hereincan be applied to other ferroelectric compositions as well as with otherelectrode structures and materials.

Utilizing the multi-layer PZT structure in conjunction with the two stepanneal process enhances ferroelectric performance by establishingsufficient lead in the nucleation layer to adequately establish theintermetallic phases of lead platinum (Pb—Pt) and lead titanate (Pb—Ti).These phases provide the foundation and seeding for proper PZTnucleation with orientation for optimum ferroelectric performance. Thenucleation layer is kept thin (about 100 to 200 Angstroms) and lead rich(30%±5%) and provides the lead necessary to complete the phaseinteractions for the time and temperature window of the argon portion ofthe anneal. The argon anneal step is desirable to enhance the Pb—Pt andPb—Ti interactions without the presence of oxygen. Typically,oxygen-only anneals overrun these interactions with the formation of PbO(lead oxide), which disrupts the preferred phase formations. The PbOformation is detrimental to preferred crystal orientation (<100> and<110> mixed orientations). After the argon portion of the anneal, thefoundation is set for good crystal orientation, <111> and <001>, but thelattice remains oxygen deficient. The second anneal is done in an oxygenambient atmosphere to complete the crystal formation, which is nowdriven by the foundation established with the argon anneal. The oxygenanneal fills the oxygen vacancies and completes the crystallinestructure for the bulk of the film. The bulk of the PZT film can not beas lead rich (bulk layer lead =15%±5%) as the nucleation layer orexcessive amounts of PbO would be generated during the oxygen anneal andagain disrupt preferred crystal orientation in the bulk of the film.

Process Flow Details

PZT is deposited on a bottom electrode consisting of a 200 Angstromtitanium sticking layer and 1750 Angstrom platinum layer. The PZTdeposition is done in a Gryphon Deposition Tool, modified for RF. ThePZT film utilized consisted of a proprietary “CS” composition(Pb/Zr/Ti/La/Ca/Sr) and is deposited using a multi-layer approach.However, the benefits of the multi-layer film structure and annealsequence of the present invention is not exclusively limited to the “CS”composition referred to above. PZT doped with only lanthanum showsimprovement using the method of the present invention. The “CS” PZTcomposition is described in further detail in co-pending patentapplications assigned to the present assignee having Ser. Nos.08/620,799 and 08/861,674, both of which are entitled “Use of Calciumand Strontium Dopants to Improve Retention Performance in a PZTFerroelectric Film”, and both of which are hereby incorporated by thisreference. The nucleation layer is deposited at an energy of 700 watts,at a pressure of 18 mTorr for about five minutes, resulting in a filmthickness of about 150 Angstroms, with about 30% excess lead. The bulklayer is deposited at an energy of 1000 watts, at a pressure of sixmTorr for between 17 to 45 minutes, resulting in films from about 850 to2350 Angstroms thick with between 8% and 18% excess lead. If desired, anadditional cap layer can be deposited at an energy of 700 watts, at apressure of 30 mTorr, for about eight minutes, resulting in a film about200 to 300 Angstroms thick with about 25% to 40% excess lead. Afterdeposition of the bi-layer or tri-layer PZT film, the entire film isannealed in a AG Heatpulse 410 RTA (rapid thermal anneal) system withthe following anneal sequence:

Step 1 (Argon Anneal)

Argon ambient atmosphere;

Sixty second delay;

Ramp temperature to 625° C. at 200° C./sec;

Hold temperature at 625° C. for 90 seconds;

Turn off heat lamps and let cool to 300° C.; and

Remove from RTA chamber.

Step 2 (Oxygen Anneal)

Oxygen ambient atmosphere;

Sixty second delay;

Ramp temperature to 750° C. at 125° C./sec;

Hold temperature at 750° C. for 20 seconds;

Turn off heat lamps and let cool to 300° C.;

Remove from RTA chamber.

After annealing, the wafer receives a top electrode deposition (1750Angstroms of platinum), patterning and etch followed by a furnace annealprocess at 650° C. for 60 minutes in an oxygen ambient atmosphere, atwhich point the wafer is ready for electrical testing.

PZT Thin Films Crystallized in reduced O₂ Partial Pressure Ambient

The main object of this embodiment of the present invention is theapplication of reduced O₂ partial pressure ambient during PZT thin filmcrystallization. Crystallizing PZT thin films in reduced O₂ partialpressure ambient results in better ferroelectric performance, crosswafer uniformity and wafer-to-wafer repeatability compared with pure O₂crystallization ambient.

Ferroelectric PZT [Pb(Zr,Ti)O₃] thin film is one of the key componentsin FRAM® technology. After being deposited on a substrate, the PZT thinfilm needs to be annealed at elevated temperature, such as 600° C., toform a polycrystalline thin film with a complex perovskite structure.Since PbO is very volatile at high temperature, say above 500° C., thelead loss in the area close to the surface of the PZT film is higherthan that in the bulk of the PZT film, resulting in a non-uniformdistribution of lead in the direction of the thickness of the film.Adding excess lead in the PZT film before crystallization may compensatethe lead loss in the surface area during high temperature anneal andform the perovskite phase. However, the lead content in the bulk of thePZT film may be too high. It may be difficult for lead to diffuse out ofthe film as the perovskite phase may block the migration of leadcations. Since the Pb—O bonding in PbO is much weaker than that in aperovskite structure, it is therefore more difficult for lead cations tomigrate through a perovskite phase. One way to increase the mobility oflead cations in a perovskite phase is to generate oxygen vacancies inthe perovskite structure. It is known that the perovskite structure cantolerate vacancy concentration up to 20%. It is therefore possible tocrystallize as-sputtered PZT film in a reduced O₂ partial pressureambient so that the perovskite structure can still form but with a largeamount of O₂ vacancies embedded in the structure. O₂ vacancies provideeffective paths for lead cations to migrate in the film during hightemperature sintering, giving rise to a more uniform lead distributionand more homogeneous formation of a perovskite phase.

There are two ways to generate a reduced O₂ partial pressure ambient.One is to use vacuum environment and flow a small amount of O₂ gas intothe annealing chamber. Another way, according to the present invention,is to simultaneously flow O₂ and another type of gas that will not reactwith PZT film, such as argon, into the annealing chamber during hightemperature crystallization. Method one requires an expensive pumpingsystem and a good vacuum control system. Method two according to thepresent invention only requires mass flow controllers and a reasonableseal of the annealing chamber.

The assignee of the present invention, Ramtron International Corp. hasused Argon rich ambient gas to crystallize PZT films for FRAMapplications and parts containing PZT film annealed in an Argon/oxygenambient atmosphere have been built and evaluated. The oxygen partialpressure in Ramtron's RTA (Rapid Thermal Anneal) chamber is in the rangeof 10⁻⁴ to 100 Torr.

Experiments have been conducted to evaluate the ferroelectricperformance of PZT films crystallized in the Argon rich ambientaccording to the present invention (Ar/O₂ mixture followed by O₂anneal—herein Ar/O₂ then O₂), and for comparison, O₂ only ambientfollowed by O₂ anneal—herein O₂ then O₂. FIGS. 2(a), (b), (c), and (d)show the comparison of switched charge V(90%), switched charge loss, andopposite state charge aging rate for the two types of PZT films it canbe seen that the PZT films crystallized in the combined Argon and O₂ambient of the present invention (i.e., Ar/O₂ then O₂) indeed showbetter ferroelectric performance compared with samples crystallized inthe O₂ ambient (i.e., O₂ then O₂) (that is, higher switch charge(Q_(SW)), lower V₉₀%, lower Q_(SW) loss @ 10⁹ cycles, and comparableopposite state switched charge aging rate).

A PZT ferroelectric thin film annealed according to the presentinvention had a five-volt switched charge of greater than 35 μC/cm², aV(90%) of less than 3.5 volts, a five-volt switched charge loss at 10⁹switching cycles of less than 45 percent, and an opposite state switchedcharge loss rate about −5 percent.

FIG. 3 is a diagram of an RTA (Rapid Thermal Anneal) chamber forannealing a PZT wafer according to the present invention. An RTA chamber32 includes heating coils 36 powered by a heating control unit 44. TheRTA chamber 32 further includes a substrate or platform 38 forsupporting a PZT wafer 40. A thermocouple 42 provides feedback toheating control unit 44. The RTA chamber 32 further includes a door 34for introducing the wafer to the chamber. Annealing gasses are providedthrough oxygen cannister 46, nitrogen cannister 48, and oxygen cannister50, which are introduced to the RTA chamber 32 under control of valves52, 54, and 56, as required.

It is important to note that the combined argon and oxygen anneal of thepresent invention can be used on either single layer, bi-layer, ortri-layer PZT films. Also, other inert gasses besides argon can be used,such as nitrogen or helium, or the like. The exact temperature andpartial pressure of oxygen can be changed as required. A second annealin oxygen can also be performed.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. For example, if desired, themethod of fabricating a ferroelectric multi-layer thin film according tothe present invention can be modified to include only a single PZTlayer, followed by the steps of annealing the single PZT layer in anargon ambient atmosphere, and then annealing the single PZT layer in anoxygen ambient atmosphere. Improvements in performance will still beobtained, even though a single PZT layer is used. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

We claim:
 1. A method of fabricating a ferroelectric thin filmcomprising: forming a PZT film on a substrate; annealing the PZT film ina combined argon and oxygen ambient atmosphere during PZT thin filmcrystallization; and thereafter annealing the PZT film in an oxygenambient atmosphere.
 2. The method of claim 1 in which forming a PZT filmcomprises forming a single layer PZT film.
 3. The method of claim 1 inwhich forming a PZT film comprises forming a bi-layer PZT film.
 4. Themethod of claim 1 in which forming a PZT film comprises forming atri-layer PZT film.
 5. The method of claim 1 in which the firstannealing step is performed at a temperature above 500° C.
 6. The methodof claim 5 in which the first annealing step is performed at atemperature of about 600° C. to form a polycrystalline thin film with acomplex perovskite structure.
 7. A method of fabricating a PZTferroelectric thin film comprising the steps of: simultaneously flowingO₂ and a second gas that does not react with PZT into an annealingchamber during high temperature crystallization of the PZT ferroelectricthin film; and thereafter flowing O₂ alone into the annealing chamber.8. The method of claim 7 in which the second gas is argon.
 9. The methodof claim 7 in which the second gas is nitrogen.
 10. The method of claim7 in which the second gas is helium.
 11. The method of claim 7 in whichthe second gas is an inert gas.
 12. The method of claim 7 in which theoxygen partial pressure in the annealing chamber during said hightemperature crystallizaton is in the range of 10⁻⁴ to 100 Torr.
 13. Themethod of claim 7 in which forming a PZT film comprises forming a singlelayer PZT film.
 14. The method of claim 7 in which forming a PZT filmcomprises forming a bi-layer PZT film.
 15. The method of claim 7 inwhich forming a PZT film comprises forming a tri-layer PZT film.